Add metadata_builder helper class

[abellina]

This PR introduces a helper class metadata_builder within the cudf::detail namespace, that allows pack and contiguous_split to create metadata for a packed buffer without having access to metadata internals (e.g. serialized_column).

The class makes it possible for callers to build metadata independent of the current implementations in pack, that are relying on column_views being defined (actual valid cuDF columns). The chunked pack work will use this class to create metadata without needing to create a column_view in the first place, since it doesn't have a valid base pointer for the columns to be instantiated.

[abellina]

I have tested with this patch in the past, and I know the cpp tests passed before I posted the PR. I had a linker error with pyarrow when it was trying to dlopen one of its internal libraries (so not even to the tests). As a result I have been recreating the conda environment and still waiting on that. Marking this as reviewable for now as I think getting some 👀 should be good and I don't expect new issues to show up in the python tests.

[abellina]

I have tested with this patch in the past, and I know the cpp tests passed before I posted the PR. I had a linker error with pyarrow when it was trying to dlopen one of its internal libraries (so not even to the tests). As a result I have been recreating the conda environment and still waiting on that. Marking this as reviewable for now as I think getting some eyes should be good and I don't expect new issues to show up in the python tests.

Local build finished and test_pack passed (CI also passed..)

[abellina]

Thanks @mythrocks, updated.

[mythrocks]

Sorry for the slow review. I'll look through this some more tomorrow.

[hyperbolic2346]

Mostly const additions requested.

[mythrocks]

LGTM!

[abellina]

Thanks for the reviews so far @mythrocks and @hyperbolic2346. Let me know if there's anything else @hyperbolic2346

[abellina]

/merge